Psychiatrists are frequently called upon to make assessments of the rationality or irrationality of persons for a variety of medical-legal purposes. A key category is that of evaluations of a patient's capacity to grant informed consent for a medical procedure. A diagnosis of mental illness is neither a necessary nor a sufficient condition for a finding of incompetence. The notion of competency to grant consent, which is a mixed psychiatric-legal concept, shares some features with philosophical conceptions of rationality, but differs from them in a number of important respects. This article describes the actual practice of psychiatrists when making such judgments, along with the standards of competency they employ. A comparison is made between those notions of competency and predominant philosophical conceptions of rationality.